Upper limit on the η → π ⁺ π ⁻ branching fraction with the KLOE experiment

Based on an integrated luminosity of 1.61 fb⁻¹e⁺e⁻ collision data collected with the KLOE detector at DAΦNE, the Frascati ϕ-factory, a search for the P- and CP-violating decay η → π⁺π⁻ has been performed. Radiative ϕ → ηγ decay is exploited to access the η mesons. No signal is observed in the π⁺π⁻ invariant mass spectrum, and the upper limit on the branching fraction at 90% confidence level is determined to be ℬ(η → π⁺π⁻) < 4.9 × 10⁻⁶, which is approximately three times smaller than the previous KLOE result. From the combination of these two measurements we get ℬ(η → π⁺π⁻) < 4.4 × 10⁻⁶ at 90% confidence level.